Metal blasting material and method of producing the same



Dec. 26, 1939. G. H. KANN 2,184,926

METAL BLASTING MATERIAL AND METHOD OF PRODUCING THE SAME Filed July 28,1938 2 Sheets-Sheet 1 3nnentor ,Gusfal/e H. Kan/7..

(Ittornegs.

G. H. KANN Dec. 26, 1939.

METAL BLASTING MATERIAL AND METHOD OF PRODUCING THE SAME Filed July 28,1958 2 Sheets-Sheet 2 Zhwentor Gusfave h. Kan/7. Mr

(Ittomegs memes Dec. 26, 1939 METAL BIASTm-G MATERIAL AND METHOD OFPRODUCING THE SAME Gustave H. Kann, Pittsburgh,- l'a., assignor to TheGlobe Steel Abrasive Company, Mansfield, Ohio, a corporation of OhioApplication July 28,1988. Serial N- 2213.! 8 Claims. (Cl. 51-280) Thisinvention is directed to metal blasting material and method of producingthe same. The material in question is particularly applicable for use inmachines generally known in the industry as metal blasting machines or"sand blasting" machines. The material is most applicable for use inmachines wherein the metal blasting material is applied by mechanical 10means rather than by air pressure to the surface to be treated. However,the material is not limited to use in the mechanical machines and may beused in the machines which operate by means of an air blast.

Itisoldinthearttousesandasamedium for cleaning metal surfaces such asthe surface of castings or the surfaces of metals which have been heattreated and are coated with a scale. It is also old in the art to usemetal shot and Q0 crushed metal particles or metal grit. There arecertain difllculties which are experienced with the materials which havebeen used up to the present time and this invention seeks to overcomesome of those difllculties. Shot made of cast iron analyses by variousprocesses have been extensively used in the industry. Such shot areusuallychilled rapidly and have a white iron structure. Crushed materialmade of chilled cast iron shot, or other cast iron products, have beenused for metal cleaning both in the air-blast equipment and in themechanically operated equipment. Likewise, crushed steel and fineparticles of steel made by several processes have been used for metalcleaning. For example, it is known that the trimmings made fromsharpening nails have been 'used for this purpose. In the case of thesevarious materials there are objectionable features. and some of thesewill be mentioned hereinafter.

40 In the case of chilled iron shot, which are prepared by the usualcommercial methods, the shot which are hard and brittle tend to crush upor explode when used in present practice, particularly when used in themechanical metal blasting equipment. Breaking up and exploding of theshot causes the product to become too finely divided for satisfactoryperformance and therefore uses excessive amounts of themetal blastingmaterial. The line material resulting from crushing of the shot must beremoved from the system and new material added and this requires timeand delays the work. Such hard material as chilled iron shot alsoincreases the wear on the equipment used in this operation. It istherefore one of the objects of this invention to produce a much toughershot, that is, one thatis not fractured in operation and it is of someimportance to produce a shot which does not wear the equipment sorapidly. For this type of work.

shot ofsomewhat diiferent-degrees of hardness are indicated; thus, it isan object of this invention to be able to produce shot of controlleddegrees of hardness but at the same time of sufg ilcient toughness thatthe shot does not fracture in service. Cost of the material is animportant consideration and for that reason cast iron is one of the mostsuitable materials. The cast iron may be in m the form of rounded shot,produced by the commercial processes which are now in use or it may bein the form of crushed material or "gritf, which is made by such anoperation as crushing the large particles of shot which are too large 15for use in the present processes. Other forms of chilled iron may alsobe crushed and used.

In the accom g drawings: Figure l is a ew of the microstructure of shotat a magnification of 100- diameters.

Figure 21s a view of the microstructure at 1000 diameters. t

Figure 3 shows the microstructure of one form of applicant's shot at,1000 diameters after heat treatment. a

Figure 4 is another illustration of the micro- 25 structure of anotherform of applicant's shot at 1000 diameters after heat treatment.

The chilled iron shot as now produced is quite hard and brittle and themicroscopic examination of such cast iron shot shows that the structureis largely made up of a hard microconstituent martensiteand a stillharder microconstituent, cementite, which is a compound of iron andcarbon. Both of these microconstituents are a brittle and this isespecially true of the carbides or the structure known as cementite. Itis, therefore. an object of this invention to change this microstructureand thereby produce a shot which has a much greater degree of toughnessbut with the hardness controlled within certain i r desired ranges.

The microstructure of certain prior art chilled iron shot is shown inFigures 1 and 2 of the accompanying drawings. This lot of shot, whichmay be designated as Lot or Sample No. 2, has a chemical composition, asdetermined by analysis, of 3.37 per cent C, 0.40 per cent Mn, 1.62 percent Si, 0.44 per cent P, 0.09 per cent Sand the remainder principallyiron. This particular lot 50 of shot was of such selected sizes that thediameters range from 0.10'to 0.108 inch. The mlcrostructure representedin Figure l is at a magnification of diameters and shows more or lesstempered martensite in the dark areas and 55 principally cementite oriron carbide in the light areas. The microstructure is more clearlyshown inFigure 2 at a magnification of 1000 diameters in which the darkareas are marten'site with some retained austenite while the white areasare ceso mentite or iron carbide. These structures are known to be hardand brittle.

In the prior art, one patentee has su gested that iron or steel shotmight be subjected to heat treatment processes to vary the hardnessthereof ii. desired. However, not all iron or steel shot can beeifectively treated by the particular heat treatment processes utilizedby me in the performance of this invention. Moreover, the term heattreatment processes is an extremely broad term and not all heattreatment processes will be effective to produce the results attained byme,

even though the chemical composition of the shot subjected to treatmentwould be suitable for use with my process. 4

It is one of the objects of this invention to treat certain. prior artshot or other shot. of proper chemical composition so as to transformthe brittle structures characteristic thereof into tougher structuresand, at the same time, to control the hardness of the shot withincertain preferred ranges.

Another object of this invention is to provide a simple and effectivemethod for treating shot of selected chemical composition so as totransform the brittle structures characteristic thereof into tougherstructures and, at the same time, to control the hardness of the shotwithin certain preferred ranges.

It is another object of this invention to produce shot for. the purposesindicated which will be distinctly superior to prior art shot both fromthe standpoint of hardness and toughness, which will have superioreffectiveness and longevity in use and which will at the same timedecrease the wear upon the blasting apparatus utilized.

Other objects of this invention will appear as this descriptionprogresses and by reference to the appended claims.

While the term "shot" is used throughout this specification, it is to beunderstood that this term is intended to include irregularly shapedparticles made by crushing or by crushing and grinding chilled iron orchilled iron particles which is generally referred to in the trade asfgrit.

My process contemplates the treatment-of shot of selected chemicalcomposition by subjecting them to temperatures and other conditions oftreatment which will effect partial malleableizing thereof or which willcompletely malleableize them. It also contemplates subjecting them totemperatures and conditions which will partly malleableize such shot andfollowing such operations with a quenching operation.

The chemical composition of chilled iron shot as supplied to industryfor metal cleaning or blasting varies considerably as is illustrated inTable I.

TABLE I.Chemical composition, of chilled iron shot 7 Chemicalcomposition Lot No.

0 Mn Si P S 3. 31 44 2.02 44 .075 3. 37 40 1.62 44 09 3. 47 37 l. 42 51095 3 47 35 l. 53 34 075 3 53 42 1. 83 39 085 31. 39 37 1.86 56 088 3 49.45 1. 90 36 089 3. 29 34 l. 61 52 084 3. 46 44 1.99 .49 158 3. 44 40 2.09 55 164 3. 48 47 1. 89 49 164 3. 33 39 2. 03 53 164 The above tableshows the following ranges in chemical compositions: Carbon 3.29 to 3.53per cent, manganese 0.34 to 0.47 per cent, silicon 1.42 to 2.09 percent, phosphorus 0.343 to 0.559 per cent and sulfur 0.075 to 0.164 percent. These ranges are only representative of a series of samples usedin some researches and not limiting values. In fact, much wider rangesin chemical composition can be used in the production of chilled ironshot and will respond to heat treatment to improve the toughness and tocontrol the hardness. The lower limit of carbon is fixed in part bymelting practice and the stock commercially available. As the carboncontent is decreased the temperature used in melting the charge must beincreased. Because of this relation and the cost of melting stock, thelow carbon limit is about 2.5 per cent for most purposes but carboncontents as low as 2.0 percent can be used. The upper carbon content islimited by the tendency of iron-carbon-silicon alloys to form graphitein the metal and thus weaken the material. Also, if the carbon contentis too high graphite forms in the shot during solidification even whenthe shot is cooled rapidly as by a stream of water. Still other methodsof producing chilled iron shot may be used in the production of shotsuitable for use in carrying out my heat treating process. It is onlyessential that the shot be produced with a chilled iron structure andthat the rate of cooling and the chemical composition be such that flakegraphite is avoided or present in only a small amount.

These conditions limit the maximum carbon' content to about 4 per centand if the silicon content is on the high side of the range then thecarbon content must be held lower, as for example, at 3.0 to 3.5 percent. The silicon content can be varied over a rather wide range, butits range is related to the carbon content. Silicon favors the formationof graphite during solidification and on cooling. It also favors theformation of graphite or temper carbon on reheating and too much siliconmay cause undesirable results in heat treating the hard brittle shot toincrease the toughness. In practice, it is generally necessary to usethe low range of silicon with the high range of carbon and when thecarbon is low the silicon content may be increased. While the averagesilicon content in the examples given in Table I is about 1.8 per cent,the maximum 2.09 per cent and the minimum 1.42 per cent, the possiblerange is much greater. For example,.in an iron of high-carbon contentthe silicon may be as low as to 1 per cent while with an iron oflow-carbon content the silicon may be as high as 2% to 3 per cent.

The carbon and silicon are of primary importance in making white ironand iron that canbe malleableized. Therefore, it is essential to havethe carbon and silicon contents so adjusted and controlled thatessentially white iron is produced by the granulating and coolingprocess and that the iron will be capable of being malleableized. Highercarbon and silicon contents may be used with good results when thegranulating and cooling is effected by pouring the molten metal fromabove into a ribbon-like stream of water under approximately pounds persquare inch pressure which thereupon delivers it into a pool of water,than when a blast of steam is used for granulation.- Thus in carryingout my invention it is possible to use molten iron containing about 1.0to about 3 per cent tents may vary over considerable ranges. While themanganese contents listed in Table I range from 0.34 to 0.47 per cent,the range may be considerably greater, especially higher. The manganeseaids. in producing chilled or hard shot and slows up the graphitizingreaction. It also reacts with sulfur to form manganese sulfides whichare less objectionable than iron sulfides. Thus, it is preferred toincrease somewhat the manganese content when the sulfur is high in theiron. The preferred range of manganese is about 0.40 to 0.80 per centbut both higher and lower contents maybe used without materiallyinterfering with my heat treating process.

The phosphorus'lowers slightly the melting temperature of the iron andincreases its fluidity. On the other hand, too high a phosphorus contenttends to make the chilled iron shot too brittle and may make the heattreated product too brittle for the best performance in certain types ofservice. Melting stock which is low in phosphorus may be more expensive.Thus it is deslrable to use a phosphorus content which meets as many ofthese desired requirements as pos-' sible and such ranges'asabout 0.30to 0.60-per cent are preferred. Phosphorus contents well over 1 percent'have been used in chilled iron shot with little or no loss inproperties.

The sulfur contents listed in Table I range from 0.075 to 0.164 percent. Sulfur aids in producing hard or chilled shot and the amount whichmay be present depends somewhat upon the manganese content, the higherthe man- I ganese the higher the sulfur-may be permitted to be. The costof melting stock may be higher if a low sulfur content is specified. Thekind of coke or other fuel used in melting may influence the sulfurcontent. In general, a low sulfur content such as .05 to .08 per cent issatisfactory but higher sulfur contents such as 0.15 to 0.20 per cent donot seem to be particularly harmful in applying my heat treatingprocess.

The data in Table I do not show the presence of such elements aschromium, nickel, copper and.

molybdenum. These elements are frequently present in the scrap used inthe melting charge and as a resultare found in the shot. More detailedanalyses than those reported in Table I have shown the presence ofchromium, copper, nickel and molybdenum in the shot. The presence ofsmall amounts of these elements does not seem to interfere with my heattreating process for toughening chilled iron shot or crushed materialmade from chilled iron.

. chromium is quite satisfactory and it is considered good practice inthe production of iron shot for metal blasting to have'present smallamounts of chromium such as up to 0.10 per cent. For shot which are tobe retained in a relatively hard condition without having to resort torapid cooling in the heat treating condition, chromium is desirmight beused for this purpose, chromium is less expensive.

Laboratory tests in shot from Lot 2 in Table I s Brinell hardness was223 and specimens required able and may be used in amounts from about0.05 to 0.50 per cent. Other carbide stabilizers but as a rule showed aBrinell hardness of about 680 and a resistance to crushing in staticcompression of 726 pounds. when tested under repeated impact blows itwas foundthat an average of 6.7 blows with a hammer weighing 4.98 poundsand falling a height of 0.67 inch was required to fracture the shot ofapproximately 0.100 to 0.108 inch diameter. These data show that shotwith microstructures such as'shown in Figures 1 and 2 possess highhardness and high compressive strength in static loading but that theyare brittle when subjected to. impact as they would be in metal cleaningby mechanical or air blasting.

By applying my heat treatment-to samples of shot from Lot 2, I have beenable to produce shot 20 of a wider range in hardness and especially to.produce shot with greatly improved resistance to fracturing whensubjected to impact. Of the many heat treatments studied only a few willbe given as illustrations.

- Example 1.A sample of shot from Lot 2 was heated for one hour at 1500"F. and then quenched in water. The Brinell hardness was 460. These heattreated shot required a load of 1000 pounds to fracture in staticcompression and withstood 80 over 200 hammer impacts without fracturing.The microstructure had been materially changed and was estimated toconsist of the following in approximate percentages: troostite 72,cementite, 15, graphite 12 and ferrite 1. The microstructure isillustrated in Figure 3.

Example 2.The sample of shot from Lot 2 was heated one-fourth hour at1500 F., one hour at 1400 F. and then water quenched. The

more than 200 hammer blows to fracture. The

microstructure was estimated as ferrite 66 per cent, graphite per cent,troostite 10 per cent and cementite 4 per cent. This heat treatmententirely removed the massive areas of cementite .and produced a productwith good resistance to impact.

Example 3.In this heat, the shot were heated one hour at 1500 F., onehour at 1400 F. and then cooled in air. The Brinell hardness was 185 andspecimens were not fractured by 200 hammer blows. The microstructure wasestimated to consist of approxlrnatelyper cent ferrite, 18 per centgraphite, 5 per cent pearlite and 2 per cent cementlte.

Example 4.Another sample of Lot 2 shot was heated three hours at 1600F., cooled at a rate of F. per hour to 1400" F. and then cooled with thefurnace to about room temperature. Specimens from this test showed aBrinell hardness of and required an average of 88 hammer blows to causerupture. By this treatment a'very soft shot was produced in which themicrostructure consisted almost entirely of ferrite and graphite ortemper carbon.

Example 5.-This sample of shot from Lot 2 was heated at 1350 F. for fourhours and then quenched in water. Specimens showed a Brinell hardness of146 and required an average of 141 hammer blows to cause fracturing. Themicrostructure showed approximately 78 per cent ferrite, 20 per centgraphite and 2 per cent cementite.

Example 6.This sample of shot from Lot 2 75 was heated at 1400 F. forone-half hour, cooled to 1350 F., and held one hour, being then waterquenched. The resulting structure as shown in Figure 4 consisted of aferritic matrix, a large amount of massive cementite and dark areas ofgraphite. Specimens showed a Brinell hardness of 228 and required anaverage of 109 hammer toughness of the material can be greatly improved.

Fundamentally, my heat treating process involves heating chilled ironshot'of selected composition, such as those previously mentioned, to

temperatures in the range of about 1300 F. to about 1600" F. for asuflicient period of time to break up the massive carbide or cementiteand then cooling at a rate selected to produce the desired hardness. Thetime required decreases as the temperature is increased. For example,about four hours is required at 1350 F. to produce a shot which will besoft and tough on slow cooling while similar results can be obtained at1600 F. in one-half hour andat 1500 F. in about one hour. Shorterheating times at these temperatures will give a product of somewhathigher hardness. In general, I further control the hardness of my heattreated shot by the rate of cooling from the high temperature. If

high hardness, such as 450 Brinell is desired,

then I find it desirable to cool the shot rapidly, as by quenching. Forintermediate hardness I 0001 the shot more slowly as in an air blast, instill air or evenin the furnace. By these methods it is practical toproduce metal blasting materials of a wide range in hardness but in allcases with improved resistance to fracturing when subjected to impact asin service. For example, all of the materials listed in Table I wereheated one-half hour at 1500 F., cooled to 1400 F., held at thattemperature for one hour and then quenched in water. All "of these lotsexcept 7, 12

and 16 withstood 200 hammer blows without fracturing whereas before theheat treatment these same materials had fractured at about 2 to 12hammer blows. There was a marked improvement in the impact resistanceover the other lots as shown by the fact that Lot 7 increased frbm 3.?to 112, Lot 12 from 5.6 to 84 and Lot 16 from 1.8 to 154 hammer blows tocause fracturing.

In addition to laboratory tests, field and service tests have been madeon shot heat treated according to my invention. In one test two thousandpounds of shot, representing a lot of one thousand pounds-of shot in thesize range of about .047 to .056" and another lot of one thousand poundsin the range of about .068 to .078" diameter were heat treated accordingto my invention and used in a practical test.

These field tests showed conclusively that the heat treated shot producemuch less wear on the mechanical equipment and that they last muchlonger in service than the usual commercial chilled iron shot.

, or its decomposition product martensite.

lic materials are used it is quite common in the trade to refer to theoperation as sand blasting". As indicated earlier in this specificationand illustrated in the examples, I control the proper- .ties andmicrostructures of the metal blasting material in part, by choosing thecomposition of the original product and in part by the heat treatmentafter granulation. It is desirable to select the composition of thematerial so that by the process of cooling the molten material I producea product which is white or chilled iron; said product-having amicrostructure consisting essentially of cementite or carbides andaustenite This product should be essentially free from graphite andshould show a white fracture. My heat treating process consists ofseveral essential features. First, there is a heating and holding attemperature which is necessary to break up the massive carbides or toput them into solution or to do both.

At the low temperature I largely break up the carbides by decomposingthem to form graphite or temper carbon. At the high temperatures I alsoeffect partial decomposition of the carbides into graphite or tempercarbon but I also eliminate the massive carbides to some extent bydissolving them in the austenite which forms at these high temperatures.

At the lowest temperatures which may be used in my process the rate ofcooling has comparatively little efiect on the hardness. At the highertemperatures, however, where there is considerable carbon in solutionthe rate of cooling has a pronounced effect on the resulting hardness asshown in the examples. the composition represented by Lot 2 for one hourat 1500 F. and then quench in water, a

' Brinell hardness of 460 is obtained. The higher the'quenchingtemperature and the more rapid the rate of cooling, the higher thehardness will be. If a material is quenched from a lower temperaturethan 1500 F. the hardness will be lower. This is illustrated in ExampleNo. 2, in which the material was quenched from 1400" F.

If a material of intermediate hardness is desired, I may, for example,heat to a relatively high temperature and then use a less rapid coolingrate as by cooling in air. Various modifications of these heattreatments are within the scope of this invention. For example, thematerial may be heated for one hour at 1500 F., cooled rapidly by waterspray to a black heat and then permitted to cool more slowly. Such If Iheat a material ofa heat treatment has certain advantages in thatperature in order to produce a high hardness such as 400 to 500 Brinelland then reheat to a lower temperature in order to increase thetoughness somewhat without greatly reducing the hardness. Still othermodifications of the heat treating process might be employed and areconsidered within the scope of this invention. For example, the abrasivematerial may be quenched in a molten lead bath to rapidly cool thematerial to a temperature below a red heat and yet avoid a drasticquench. The quenching processes which have been described in theliterature as austempering in which the quenched product is caused totransform from the high-temperature modification to another modificationat selected temperatures thereby obtaining selected hardnesses andmicrostructures may be utilized in the performance of my process.

Some of the unique features of my process are the fact that I selectshot of such a chemical composition that they may be either partially orcompletely malleableized and that the hardness thereof may be variablycontrolled by various quenching operations. An important factor is thatI am able to obtain shot having various desirable properties bysubjecting chilled cast -iron shot of selected materials and subjectingthem to selected heat treating processes. Cast iron is a relatively.cheap material and yet my process permits of obtaining shot therefromwhich possess more desirable properties than more expensive materials.Other characteristics of my new abrasive material will be evident fromthe appended claims.

Having thus described my invention, what I claim is:

1. A metal blasting material in the form of shot or grit, said materialcontaining carbon in the range of 2.0 to 4%, silicon from .5 to 3%,manganese from .20 to 2%, sulfur f'romtraces to '.3%, phosphorus from.05 to 2% and the balance substantially iron, and said material havingbeen at least partially malleableized from white iron and cooled at sucha rate that its toughness is increased and its hardness is decreased towithin a range from 125 to 500. on the Brinell scale.

2. A metal blasting material in the form of shot or grit, said materialcontaining carbon in the range of 2.0 to4%, silicon from .5 to 3%,manganese from .20 to 2%, sulfur from traces to .3%, phosphorus from .05to 2%, from traces to 1% of at least one carbide former selected fromthe group consisting of chromium, molybdenum,

vanadium, tungsten, zirconium, and titanium, and the balancesubstantially iron, and said ma:

terial having been at least partially malleableized from white iron andcooled at such a rate that its toughness is increased and its hardnessis decreased to within a range from 125 to 500 on the Brinell scale.

s. A metal blasting material'in the mm of shot or grit, said materialcontaining carbon in the range of 2.9 to 3.5%, silicon from 1.0 to 2.0%.manganese from .40 to .80%, sulfur from .05 to .15%, phosphorus from .10to 1.0%, and the balanc substantially iron, and said material havingbeen at least partially malleableized from white iron and cooled at sucha rate that its toughness is increasedand its hardness is decreased towithin a range from 125 to 500 on the Brinell scale.

4. A metal blasting material in the form of shot or grit, saidmaterialcontaining carbon in the range of 2.9 to 3.5%, silicon from 1.0 tov2.0%, manganese from .40 to .80%, sulfur from .05 to .15%, phosphorusfrom .10 to 1.0%, from traces to 1% of at least one carbide formerselected from the group consisting of chromium, molybdenum, vanadium,tungsten, zirconium, and titanium, and the balance substantially iron,and saidmaterial having been at least partially malleableized from whiteiron and cooled at such a rate that its toughness is increased and itshardness is decreased to within a range from 125 to 500 on the Brinellscale.

5. The method of producing metallic blasting material in the form ofshotor grit, which comprises producing white iron particles of thedesired form containing about 2.5 to 4% carbon, 0.5 to 3% silicon, .2 to2% manganese, from traces to .3% sulfur, from .05 to 2% phosphorus andthe balance substantially iron, heating said particles toa temperaturefrom.1300 to 1600 F. for a period from one-half hour to six hours andcooling the heated particles, the said heating andthe rate of coolingbeing eflective to produce at least partial malleabilization, to impartthereto a hardness between 125 and 500 onthe Brinell scale and animprovement in toughness of at least 100%, as indicated by hammer testson the material before and after heat treating.

6. The method of producing metallic blasting material in the form ofshot or grit, which comprises producing white iron particles of thedesired form containing about 2.5 to 4% carbon, 0.5 to 3% silicon, .2 to2% manganese, from traces to .3% sulfur, from .05 to'2% phosphorus, fromtraces to 1% of a carbide former selected from the group consisting ofchromium, molybdenum, vanadium, tungsten, zirconium and titanium, andthe balance substantially iron, heating-said particles to a temperaturefrom 1300 to 1600 F. for a period from one-half hour to six hoursandcooling the heated particles, the said heatingand. the rate ofcooling being effective to produce at least partial malleabilization, toimpart thereto ahardness between 125 and 500 on the Brinell scale and animprovement in toughness of at'least 100%, as indicated by hammer testson the material before

